There are a few reasons why customs officers might not want to stamp your receipts. Not all customs officers are experienced and aware of the rules and regulations surrounding VAT refunds.
It sometimes happens that a customs officer refuses to stamp a receipt if you do not have a tax free form. Please explain calmly that EU law does not state it is a rule to present a tax free from in order to get a customs stamp. Sometimes shops are not able to give their customer a tax free form. Either because they have run out of tax free form, or because they are not affiliated with a tax refund company. Many shops refund VAT directly without using an intermediary agency, but in order to refund the VAT to their customer, the receipt must be stamped by customs.
It sometimes happens that a customs officer refuses to stamp a receipt if they do not recognize your tax free form. Please explain calmly that EU law does not specify that certain tax refund agencies should be used in order to get a VAT refund. This would be against free democratic and economic principles. The fact that the customs officers does not recognize your tax free form, is not an argument for rejection of your receipt. Especially since you are not obliged to present a tax free form in the first place.
It sometimes happens that a customs officer refuses to stamp a receipt if your NCP (name, country, passport number) details are not specified on your receipt. To avoid rejection based on this reason, please write these details by hand on the back of your receipt.
If the customs officer still refuses to stamp your receipts based on this reason, please try to call the customer service line of the airport to explain, and ask if somebody is able to come down to help you. If this is not possible please try to get your receipts stamped at your point of arrival. If this is not possible, please file a complaint with the airport after your return, to see if they can ask the customs office to validate your receipts. Please send them a copy of your passport, a copy of your receipts, and a copy of your boardingpass/ticket.